Correct precedence for arithmetic operators

[toefel18]

See issue #787

Exposed did not follow the correct operator precedence rules for arithmetic operators.

[Tapac]

Thank you for the PR.
I have one extension: since the last release (0.21.1) there is CustomOperator class which suites operators like div/plus/etc very well.

Do you mind to change all the operators (where applicable) to be inherited from a CustomOperator?

[toefel18]

I'll take a look and change it to CustomerOperator! 👍 Let's see if I have some time later on today.

I would also like to change the type of .avg(), any reason why that is the only one typed BigInteger?

[Tapac]

It's BigDecimal because of some databases returns floating value as a result of AVG even on integer/long columns. (See samples here)

[Tapac]

I meant something like:

class DivideOp<T, S: T>(
    val expr1: Expression<T>, val expr2: Expression<S>, override val columnType: IColumnType
): ExpressionWithColumnType<T>()

should become

class DivideOp<T, S: T>(expr1: Expression<T>, expr2: Expression<S>, columnType: IColumnType) 
  : CustomOperator<T>("/", columnType, expr1, expr2)

[toefel18]

Sorry, I totally misread your initial comment. I updated the code and added a comment for the avg(). If you think this is common sense, I can remove it if you prefer.

Some unit tests fail at on my machine (also when I run a build without my changes)

Some failures are in the java-time module and are due to precision.

org.jetbrains.exposed.MiscTableTest > testUpdate03 FAILED
    java.lang.AssertionError: Failed on h2 expected:<2020-02-13T21:02:47.938743> but was:<2020-02-13T21:02:47.938>

But 6 failures in exposed-core seem valid, like ReplaceTests.testReplace01:
The error is:

22:24:13,294 ERROR Test worker Exposed:checkMultipleDeclaration:392 - Confusion between multiple declarations of primary key on . Use only override val primaryKey=PrimaryKey() declaration.
22:24:13,296  WARN Test worker Exposed:invoke:169 - Transaction attempt #0 failed: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Incorrect table name ''. Statement(s): CREATE TABLE IF NOT EXISTS `` (username VARCHAR(16) PRIMARY KEY, `session` VARBINARY(64) NOT NULL, `timestamp` BIGINT DEFAULT 0 NOT NULL, serverID VARCHAR(64) DEFAULT '' NOT NULL)
org.jetbrains.exposed.exceptions.ExposedSQLException: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Incorrect table name ''
SQL: [CREATE TABLE IF NOT EXISTS `` (username VARCHAR(16) PRIMARY KEY, `session` VARBINARY(64) NOT NULL, `timestamp` BIGINT DEFAULT 0 NOT NULL, serverID VARCHAR(64) DEFAULT '' NOT NULL)]

Adding a table name fixes this:

    @Test
    fun testReplace01() {
        val NewAuth = object : Table() {   // <-  missing table name?
    ...

Shall I fix these errors or am I missing something here?

[Tapac]

Which Java version do you use?
"Incorrect table name '' error looks similar to one when KClass.simpleName doesn't return a name for anonymouse classes (works fine on Java 8 but fails on 9+).

I'll check it by myself, so at the moment the PR looks fine to me.
Thank you.